1. Field of the Invention
The present invention relates to a lithographic projection apparatus and a device manufacturing method.
2. Description of the Related Art
The term xe2x80x9cprogrammable patterning devicexe2x80x9d as here employed should be broadly interpreted as referring to device that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate. The term xe2x80x9clight valvexe2x80x9d or xe2x80x9cspatial light modulatorxe2x80x9d (SLM) can also be used in this context. Generally, the pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). An example of such a patterning device is programmable mirror array. One example of such an array is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that, for example, addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind. In this manner, the beam becomes patterned according to the addressing pattern of the matrix-addressable surface. An alternative embodiment of a programmable mirror array employs a matrix arrangement of tiny mirrors, each of which can be individually tilted about an axis by applying a suitable localized electric field, or by employing piezoelectric actuators. Once again, the mirrors are matrix-addressable, such that addressed mirrors will reflect an incoming radiation beam in a different direction to unaddressed mirrors. In this manner, the reflected beam is patterned according to the addressing pattern of the matrix-addressable mirrors. The required matrix addressing can be performed using suitable electronics. In both of the situations described hereabove, the patterning device can comprise one or more programmable mirror arrays. More information on mirror arrays as here referred to can be seen, for example, from U.S. Pat. Nos. 5,296,891 and 5,523,193, and PCT publications WO 98/38597 and WO 98/33096. In the case of a programmable mirror array, the support structure may be embodied as a frame or table, for example, which may be fixed or movable as required.
Another example of a patterning device is a programmable LCD array. An example of such a construction is given in U.S. Pat. No. 5,229,872. As above, the support structure in this case may be embodied as a frame or table, for example, which may be fixed or movable as required.
For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask and mask table. However, the general principles discussed in such instances should be seen in the broader context of the patterning device as hereabove set forth.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (IC""s). In such a case, the patterning device may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In current apparatus, employing patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion at once. Such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatus, commonly referred to as a step-and-scan apparatus, each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the xe2x80x9cscanningxe2x80x9d direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction. Since, in general, the projection system will have a magnification factor M (generally less than 1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be seen, for example, from U.S. Pat. No. 6,046,792.
In a known manufacturing process using a lithographic projection apparatus, a pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. It is important to ensure that the overlay juxtaposition) of the various stacked layers is as accurate as possible. For this purpose, a small reference mark is provided at one or more positions on the wafer, thus defining the origin of a coordinate system on the wafer. Using optical and electronic devices in combination with the substrate holder positioning device (referred to hereinafter as xe2x80x9calignment systemxe2x80x9d), this mark can then be relocated each time a new layer has to be juxtaposed on an existing layer, and can be used as an alignment reference. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book xe2x80x9cMicrochip Fabrication: A Practical Guide to Semiconductor Processingxe2x80x9d, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4.
For the sake of simplicity, the projection system may hereinafter be referred to as the xe2x80x9clens.xe2x80x9d However, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a xe2x80x9clensxe2x80x9d. Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such xe2x80x9cmultiple stagexe2x80x9d devices the additional tables may be used in parallel or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Dual stage lithographic apparatus are described, for example, in U.S. Pat. No. 5,969,441 and WO 98/40791.
In apparatus using a programmable patterning device, such as an SLM, a critical issue is to ensure that the apparatus has sufficient throughput (the number of wafers processed in a given time). The throughput is determined by the rate at which the SLM can be reconfigured, which in turn is limited by the rate at which the data setting the SLM can be processed. Limits on the data processing rate arise from limits on data transfer rates, limits on data decompression and the extent to which the data must contain redundancies to ensure that the SLM is correctly reconfigured. The limit on the speed of data processing also limits the size of the SLM that can be used. Therefore, to expose the entire required image on the surface of the substrate, the image to be projected onto the substrate is divided into sections, and the SLM configured to project each section in turn. This means that the SLM must frequently be reconfigured, further reducing the throughput.
It is an aspect of the present invention to provide a lithographic projection apparatus, especially one using a programmable patterning device, that can be operated with a higher throughput.
This and other aspects are achieved according to the invention in a lithographic apparatus including a radiation system constructed and arranged to supply a projection beam of radiation; a programmable patterning device constructed and arranged to pattern the projection beam according to a desired pattern; a substrate table constructed and arranged to hold a substrate; a projection system constructed and arranged to project the patterned beam onto a target portion of the substrate, the projection system including a beam splitting unit constructed and arranged to split the patterned beam to form a plurality of patterned beam fractions; first projection optics constructed and arranged to project a first patterned beam fraction onto a first target portion of a substrate; and second projection optics constructed and arranged to project a second patterned beam fraction onto second target portion of a substrate.
This apparatus is advantageous as each substrate typically requires a plurality of copies of the same image exposed on it to form a plurality of copies of the same device. Therefore, by projecting more than one copy of the same image onto the substrate at once, the time taken to expose all the images on the substrate will be reduced, thereby improving the throughput. The apparatus is also advantageous because the multiple images can be projected onto the substrate using only a single SLM or other programmable patterning device, thereby reducing the total cost of the apparatus.
In a yet further preferred embodiment of the present invention, the apparatus further comprises a further beam splitting unit constructed and arranged to split one of the patterned beam fractions to produce further patterned beam fractions. This is advantageous since the greater the number of images targeted on the substrate, the higher the throughput of the apparatus.
According to a yet further embodiment of the present invention, the projection System comprises a beam intensity adjusting device constructed and arranged to adjust the beam intensity of the patterned beam fractions. This allows the patterned beam fractions to be adjusted such that they each have substantially the same beam intensity and therefore the portions of the substrate targeted by each patterned beam fraction will be equally exposed.
According to a yet further embodiment of the present invention, the projection System further comprises a reduction device constructed and arranged to provide a reduced image of the pattern provided by the programmable patterning device. This is advantageous since the pixel size of the SLM is larger than the critical feature size required on the substrate. The size of the image therefore needs to be reduced. Furthermore, by reducing the size of the image of each of the patterned beam fractions separately, each can be adjusted to be the correct size.
According to a yet further embodiment of the present invention, the projection system further comprises an adjusting device constructed and arranged to separately adjust the focus and/or the lateral position of the image of each of the patterned beam fractions. This ensures that the image of each of the patterned beam fractions is correctly projected on the substrate.
The beam splitting device may be a partially reflective or refractive device.
According to a further aspect of the invention there is provided:
a device manufacturing method including providing a substrate that is at least partially covered by a layer of radiation sensitive material; providing a projection beam of radiation using a radiation system; using a programmable patterning device to endow the first projection beam with a pattern in its cross-section; projecting the patterned projection beam of radiation onto a target portion of the layer of radiation-sensitive material; splitting the patterned beam of radiation into a plurality of patterned beam fractions prior to projecting it onto the substrate; and separately projecting the patterned beam fractions onto separate target portions of a substrate.
Although specific reference may be made in this text to the use of the apparatus according to the invention in the manufacture of ICs, it should be explicitly understood that such an apparatus has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms xe2x80x9creticlexe2x80x9d, xe2x80x9cwaferxe2x80x9d or xe2x80x9cdiexe2x80x9d in this text should be considered as being replaced by the more general terms xe2x80x9cmaskxe2x80x9d, xe2x80x9csubstratexe2x80x9d and xe2x80x9ctarget portionxe2x80x9d, respectively.
In the present document, the terms xe2x80x9cradiationxe2x80x9d and xe2x80x9cbeamxe2x80x9d are used to encompass all types of electromagnetic radiation, including ultraviolet radiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm) and EUV (extreme ultra-violet radiation, e.g. having a wavelength in the range 5-20 nm).